1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method, and particularly, to the apparatus and the method for polishing semiconductor wafers based on a chemical mechanical polishing (CMP) technique. The present invention also relates to a backing plate and a backing film used by the polishing apparatus.
2. Description of the Related Art
FIG. 1A is a top view showing a polishing apparatus according to a related art and FIG. 1B is a side view showing the same. The polishing apparatus is used for semiconductor device manufacturing for polishing and planarizing the steps in the surface of a semiconductor wafer due to devices and interconnections formed thereon. A disk-like surface plate 8 has a shaft 10 rotated by a driver (not shown). A polishing cloth 7 made of, for example, polyurethane foam is attached to the top of the surface plate 8. A port 11 supplies abrasive 12 onto the polishing cloth 7. A wafer base 13 is aged above the surface plate 8. The bottom of the wafer base 13 holds a wafer. The wafer base 13 has a shaft 9, which is connected to a pressing unit (not shown) and a rotating unit (not shown). The pressing unit presses the wafer against the polishing cloth 7. The rotating unit rotates the wafer in the same direction as the rotating direction of the surface plate 8.
FIG. 2 is a sectional view showing the wafer base 13 and the vicinity thereof. The wafer base 13 is made of a head 6, backing plate 4, a backing film 2, and a guide 5. The head 6 is driven by the shaft 9 and rotates above the surface plate 8. The head 6 is pushed down by the pressing unit through the shaft 9. The head 6 uniformly presses the wafer 1 against the polishing cloth 7 through the backing plate 4. To flatly polish the wafer 1, an interface of the backing plate 4 with the backing film 2 is processed flat. The backing film 2 is resilient so that the backing plate 4 may evenly press the wafer 1 against the polishing cloth 7. Even if dust is present between the wafer 1 and the backing film 2, the backing film 2 is flexible to contain the dust so that a surface of the wafer 1 to be polished may evenly be pushed against the poling cloth 7. The guide 5 prevents the wafer 1 from moving away from the backing film 2. An end face of the guide 5 that faces the polishing cloth 7 is higher than the polished surface of the wafer 1 with respect to the polishing cloth 7. When the wafer 1 is set on the polishing cloth 7, the end face of the guide 5 is away from the polishing cloth 7. When the wafer 1 is pressed against the polishing cloth 7, the backing film 2 and polishing cloth 7 are compressed and the end face of the guide 5 comes in contact with and presses the polishing cloth 7.
With this arrangement, the port 11 feeds the abrasive 12 onto the polishing cloth 7 that is rotated. The wafer 1 set under the backing film 2 is rotated and pushed by the wafer base 13 toward the polishing cloth 7 so that the surface of the wafer 1 contacting with the polishing cloth 7 is polished.
Polishing rates and their uniformity on a thermal oxidation film formed on the surface of an 8-inch silicon wafer will be explained. The wafer has LSIs formed on the surface thereof. The size of each LSI is dependent on a step-and-repeat technique used to form the LSIs and is usually 1-cm square. To improve the yield and quality of LSIs on each wafer, polishing rates within the wafer must be as uniform as possible.
FIG. 3 shows polishing rates measured at different measurement points on a wafer. The wafer is a silicon wafer of 200 mm in diameter and has a thermal oxidation film to be polished with the wafer being pressed against the polishing cloth 7 and the guide 5 being away from the polishing cloth 7. The measurement points 1 to 7 are set along a straight line passing through the notch and center of the wafer and are away from the center of the wafer by 96 mm, 80 mm, 40 mm, 0 mm, 40 mm, 80 mm, and 96 mm, respectively. Namely, the measurement points 1 and 7 are at the periphery of the wafer, and the measurement point 4 is at the center thereof. Polishing rates measured at the points 1 and 7 are each about 1.7 times greater than that measured at the point 4.
FIG. 4 shows polishing rates measured at different measurement points with the guide 5 being pressed against the polishing cloth 7 when polishing a thermal oxidation film formed on a silicon wafer of 200 mm in diameter. The measurement points 1 to 7 are the same as those of FIG. 3. Polishing rates at the peripheral measurement points 1 and 7 are about 20% smaller than those at the other measurement points. Compared with FIG. 3, FIG. 4 shows an improvement in the uniformity of polishing rates on the wafer, and therefore, it can be said that pressing the guide 5 against the polishing cloth 7 is advantageous. This, however, may deteriorate the quality of LSIs formed at the periphery of the wafer below criteria because the peripheral polishing rates are about 20% smaller than the others.